Duplex method for telegraph cables



DUPLEX METHOD FOR TELEGRAPH CABLES Filed Feb.8 1926 2 Sheets-Sheet 1 Fig.4.

In H! III! 'III! in B M 5 W an 1 C5 7 52 g CZ T 2W:

In VENTaIQ 2 Sheets-Sheet 2 frequency Dec. 27, 1927.

Patented Dec. 27, 1927.

UNITED STATES 1,653,971 PATEN HANS sALiNenn, or BERLIN, enniuiinm DUPLEX METHOD FORTELEGRAPH ,cABILEs.

Application filed February 8, 1926, Serial No. 86,955, and in Germany February 10, 1925.

Various arrangements are known for effecting duplex operation in telegraph cables or lines, in all of which use is made of the principle that the transmitting current is divided into two equal parts which balance one another in their action upon the receiver.

The invention is diagrammatically illustrated in the accompanying drawings in which Figure 1 represents an artificial cable whilst Figures 2 to 4 are explanatory diagrams For carrying the principle above referred to into efiect it is necessary to use an artificial line the behaviour of which with respect to the transmitting current is exactly the same as the behaviour of the actual cable or line. According to known methods the transmitting currents may be considered as a superposition of sinusoidal alternating currents and consequently the reproduction of the artificial line need be exact 0111 as regards that range of frequency whic corresponds to the frequency of these currents. It is already known to limit this range of frequency in the higher values 1n such a manner that higher frequencies than are absolutely necessary for telegraphic transm1ssion are omitted, so that as regards these, also the artificial line need no longer correspond. As regards the lower values the range of frequency. has not hitherto been limited since for the most usual methods of telegraphic transmission also exceedingly low frequencies and even continuous currents are employed. Consequently hitherto the artificial line has always been so constructed that the cable was balanced also with respect to continuous current (permanent current).

It is true that methods of telegraphy are also known in which the very low frequencies of approximately 1 to 2 periods per second are not utilized (curbing and transmissions according to Delany or over a blocking condenser). Thus for instance Figure 2 shows the curve of the amplitudes of a single sign, viz, of a Morse dot. This curve shows the relative amplitudes of the various frequencies. Only the part of the curve to the left of the dotted line belongs to the required band of frequencies. (See for instance Milnor, Journal A. I. E. E. 41, pages 118136 of 1922.) Figure 3 shows similarly the curve correspondlng to a method of telegraphy in which a single Morse dot consists ofa tive rectangle as indicated inthe figure. It will be seen that thelowermost frequencies have very low amplitudes and that it is per missible to neglect them (approximately on the left of the lefthand ever, it has not hitherto been recognized that it is the artificial reproduction of the cable for continuous current, which per se appears very simple,

cable are changed considerably. The leakage of the cable depends to a large extent on the frequency,,and as regards continuous current it is much smaller than, for instance, for alternating currents of 5 to 10 periods Moreover, as a rule, as regards the medium frequency of telegraphic transmission the cable has to be considered electrically so long that only its wave resistance need be artificially reproduced. On the other hand, as regards the lowest frequencies and permanent current, electrically considered the cable is not long, so that now the important factor is not the wave resistance, but the impedance, which in that case is different therefrom The impedance, however, depends to a very large extent upon the apparatus which are connected at the far end. Therefore, if we retain the lowest frequencies, not only the cable but also the ment at the far end has to be d 1 artificially repro ucec For all these reasons the retention of the lowest range of frequencies for the artificial reproduction is inconvenient and the reproduction is greatly facilitated if the said range of frequency is dispensed with. The methods to be adopted for this purpose are known. either left out when transmitting, in the manner already above referred to, or they are kept away only from the receiver.

One arrangement for carrying the invention intoeffect is illustrated by way of example in Figure 1, in which B is the transmitting battery, K, and K are the arms of the bridge, L is the cable and N the artificial line. A network of filtering circuits is inserted before the receiver, the object of which filtering circuits is to keep away from.

the receiver the undesired frequencies. As above stated, th1s network of filtering circuits may also be arranged in series with the .battery B. The artificial line N which has been illustrated in the figure is an arrangedotted line). Howa These frequencies are positive and a nega that considerably increases the difficulty of balancing, since towards the very low frequencies the properties of thearrangement which has proved to be successfulin the experiments that have been carried out in connection with-a cable having a high inductance. w 'w Q03, w, are ohmic resistances, s and s, are self-inductance coils and" c c 0, and c, are condensers. As will seen this arrangement 15 not suitable for continuous current, its impedance for low frequencies therefore differs considerably from the impedance of the actual cable. The d fl'erence between a duplex arrangement balanced according to i the known method and a duplex arrangement balanced according to the present method Wlll be seen from the curves shown in Figure 4:. The

ordinates in this figure represent the discrepancy between the artificial cable and the actual cable, and the abseissae the frequency, the scale being the same as in Figures 2 and 3; the full line shows, by way of example, the curve of the discrepancy when the known method is used and the'dotted line the curve of the admissible discrepancy for low frequencies. a

It is clear thata cable balanced according to the present invention will allow duplex I operation to be efiected for high speed telegraphy, but not for slow Morse operation. However, this is not to be considered to constitute a great disadvantage.

hat I claim is 1. Duplex method for telegraph. cables consisting in balancing the cable only with respect to the range of frequencies required for telegraphy with the exception of the 35 frequencies below two periods per second.

2. Duplex method for telegraph cables consisting in balancing the cable only with respect to the range of frequencies requireu for telegraphy but not with respect to conon. HANS SALINGETR, 

